1. Field of the Invention
The present invention relates to a butterfly valve comprising a valve shaft to be placed across a flow passage and a plate-like valve element arranged on the valve shaft, the valve element being rotatable about the valve shaft to regulate a flow rate of fluid in the flow passage.
2. Description of Related Art
Recent internal combustion engines has faced a challenge to an expansion in the range of a flow rate of air allowed to pass through an intake passage of a throttle valve in order to meet demands for increasing gas mileage and achieving high power. If a bore diameter of a part in which the throttle valve is mounted is increased, the flow rate of air allowed to pass through the bore while the throttle valve is in a full open position generally increases, enhancing output power of the international combustion engine. However, this structure would also cause an increase in an idle flow rate of air allowed to flow during an idle operation with the throttle valve held in a nearly closed position, thus deteriorating gas mileage of the internal combustion engine. For this reason, the flow rate at full open has to be increased without increasing the bore diameter in order to achieve high output power of the internal combustion engine.
As the throttle valve, conventionally, a so-called butterfly valve has been used. This type of butterfly valve is disclosed in for example Japanese unexamined utility model application publication No. 1(1989)-60080 and Japanese unexamined patent application publication No. 11(1999)-173432. The butterfly valve disclosed in the publication '080 is a conventional product, as shown in FIG. 30, which includes a valve shaft 52 placed across a flow passage 51 and a nearly disc-shaped valve element 53 arranged on the valve shaft 52 so as to be rotatable about the valve shaft 52 to regulate the flow rate of fluid allowed to pass through the flow passage 51. Herein, the valve element 53 is designed to have a uniform thickness smaller than the outer diameter of the valve shaft 52.
Further, the butterfly valve disclosed in the publication '432 includes, as shown in FIGS. 31 and 32, a valve shaft 52 placed across a flow passage 51 and a plate-like valve element 54 arranged on the valve shaft 52 so as to be rotatable about the valve shaft 52 to regulate the flow rate of fluid allowed to pass through the flow passage 51. Here, the valve element 54 has a curved surface of a curvature radius which is constant from the valve shaft 52 to an outer edge. Specifically, the valve element 54 has a section area decreasing radially outwardly from the center. Both sides of the valve element 54 is streamlined with no irregularity in three dimensions.
Since the conventional butterfly valves are made of metal in many cases, however, design freedom for shape would be low and accordingly there have been less ideas or designs to reduce air resistance. On the other hand, recently a butterfly valve made of resin has been adopted to define the direction of air flow by utilizing design freedom for shape. However, no idea has been proposed to increase the area of a flow passage.
In the butterfly valve disclosed in the publication '080, the flow line is sharply changed around the valve shaft 52 when the valve element 53 is in the full open position. This would cause turbulence of flow in a region downstream from the valve shaft 52. Accordingly, the flow rate at full open would be decreased by the flow quantity of fluid involved in the flow turbulence.
The butterfly valve disclosed in the publication '432, on the other hand, tends to cause less flow turbulence around the valve shaft 52 than the butterfly valve of the publication '080, i.e. the conventional product, particularly at full open of the valve element 54, but tends to cause a surface flow of fluid flowing radially outwardly from the center of the valve element 54. This surface flow is not parallel to the flow passage 51, which may cause flow turbulence of fluid, resulting in large pressure loss of the fluid and hence nonsmooth flow of the fluid. For this reason, the flow rate of fluid passing through the flow passage 51 at full open of the valve element 54, that is, the full open flow rate may be decreased.